1. Department of Reproductive Medicine, The Second Hospital Affiliated to Lanzhou University, Lanzhou, Gansu 730030, P.R. China
2. Department of Obstetrics and Gynecology, The People's Hospital of Gansu, Lanzhou, Gansu 730000, P.R. China
3. Department of Obstetrics and Gynecology, The Women and Children Hospital of Shaanxi Province, Xi'an, Shaanxi 710061, P.R. China
4. Department of Gynecology, The Maternal and Child Care Service Center of Dongguan City Guangdong Province, Dongguan, Guangdong 523120, P.R. China